Document MGJ9KQ1YJzjr2vQ795jgEvD9z

TEXAS BUR. OF ECON. GEOLOGY GEOL. CIRC. NO. 7 3 -1 BUREAU OF EC O N O M IC G EO LO G Y 73-1G e o lo g ic a l C irc u la r Asbestos in the Allamoore Talc District, Hudspeth and Culberson Counties, Texas BY R. G. ROHRBACHER The University of Texas at Austin A ustin, Texas 7 8 7 1 2 W . L. Fisher, Director 1973 PLAINTIFFS EXHIBIT WCD-222 BUREAU OF ECO NO M IC GEOLOGY 73-1G e o lo g ic a l C irc u la r Asbestos in the Allamoore Talc District, Hudspeth and Culberson Counties, Texas BY R. G. ROHRBACHER The University of Texas at Austin A ustin, Texas 7 8 7 1 2 W . L. Fisher, D irector -<373 CONTENTS Page In tro d u ctio n ............................................................................................................................................... 1 G e o l o g y ........................................................................................................................................................ 3 R e g io n a l s e t t i n g ........................................................................................................................... 3 A l l a m o o r e d i s t r i c t ...................................................................................................................... 4 T a l c d e p o s i t s ................................................................................................................................ 4 A s b e s t o s .................................................................................................................................................... 6 B lu e a m p h ib o le r o c k ................................................................................................................. 6 R i c h t e r i t e - t a l c r o c k ....................................................................................................... . 7 O c c u r r e n c e s ..................................................................................................................................... 9 N o r t h - c e n t r a l p a r t of d i s t r i c t .................................................................................... 9 O th e r p a r t s ofd i s t r i c t ............................................................................. O r ig in an d g e o lo g ic s i g n i f i c a n c e ................................................................................................. 13 P o t e n tia l ................................................................................................................................................... 15 R e f e r e n c e s .............................................................................................................................................. 16 ILLUSTRATIONS F igures-- Page 1. G e n e r a l g e o lo g y and a s b e s t o s d e p o s i t s , A l l a m o o r e ta lc d i s t r i c t . . 2 2. G e o lo g ic m a p and s t r u c t u r e s e c t io n of D iab lo a s b e s t o s p r o s p e c t . . 10 TABLE T ab le-- Page C h em ical a n a ly sis of c r o s s - f ib e r rich terite a sb e sto s fro m the D iab lo p r o s p e c t ......................... . . . . . . .................................................. 8 A SB E ST O S IN THE A LLA M Q O R E TA LC DISTRICT, HUDSPETH AND CULBERSON COUNTIES, TEXAS R. G. Rohrbacher^ INTRODUCTION The A llam oore d istrict of Hudspeth and Culberson counties, Texas has becom e one of the m o st significant talc-producin g a r e a s of the United States. Exploitation of talc d e p o sits in the d is tr ic t began in 1952 with a cum ulative production of 120, 000 tons through 1957 (Flaw n, 1958). With continued growth, annual production exceeded 1 6 0 ,0 0 0 tons in 1968, m ak in g the d i s t r i c t se c o n d only to New Y o rk S ta te in n atio n al output. M o d e ra te - to l a r g e - s iz e d d e p o sits have been developed with n e a r - s u r f a c e p arts inexpensively extracted. Talc r e s e r v e s a r e estim ated in the tens of m illions of tons. L o n g-fib er a sb e sto s w as fir s t found in asso c ia tio n with talc dep osits of the A lla m o o r e d i s t r ic t in I960 during e x p lo r a to r y d rillin g of what b e c a m e the B uck claim . Sm all am ounts of a sb e sto s w ere noted in subsequent developm ent of this larg e talc deposit. L a te r , sm all am ounts of white a sb e sto s w ere encountered in the T. & P. No. 1 m ine and recen tly at the N eal-M an n p r o s p e c t (fig. 1). E a r ly in 1971, A lbert G regory of Van Horn, T e x a s, d iscovered an a sb e sto s deposit now known a s the Diablo p ro sp ect. Subsequently, the Van Horn Soapstone and Talc Corporation w as form ed and prelim in ary exploratory work at the p rosp ect indi cated the p ossib le p resen ce of com m ercially exploitable amounts of a sb e sto s. 19871 M cKean Road, San Jo se , California 95120. 1 to F I G . 1. G e n e r a l g e o l o g y a n d a s b e s t o s d e p o s i t s , A l l a m o o r e t a l c d i s t r i c t 3 T his evaluation of a s b e s to s d e p o sits is b a se d on a cu rre n t study of the s t r a tigraphy, structure, and m in eral deposits of the talc d istrict. A subsequent report w ill detail the m in eralogy of m etaso m atic rocks of the d istric t, including talc and asbestos deposits, and will present a geochem ical model for their origin. GEOLOGY Regional Setting T alc and a s s o c ia t e d a s b e s t o s o c c u r in P r e c a m b r ia n ro c k s in a d efo rm ed belt prev io u sly d escrib ed by King and Flaw n (1953) and P. B. King (1965). F r o m n orth to south, the gently dipping but stro n g ly join ted san d ston e and conglom erate of the H azel F orm atio n change within se v e ra l m ile s into a se v e re ly deform ed m ixture of H azel F orm atio n and the carbonate and volcanic rock s of the A lla m o o r e F o r m a t io n (fig. 1). A d jo in in g on the south a r e the m e t a s e d im e n t a r y and m etaigneous rocks of the C a rrizo Mountain Group. The rocks of these three divisions grade from practically unmetamorphosed in the n orth ern p a rt to g r e e n s c h is t and am phibolite fa c ie s in the southern p art. M etam o rp h ism of the cen tral division w as m ostly dynam ic, with v aried d e g re e s of re c ry sta lliz a tio n and m e ta so m a tism taking place p rim a rily in carbonate ro ck s. M in eral asso c ia tio n s in the cen tral division su g ge st that m etam orph ic grade in c r e a s e s southw ard, re p re se n tin g p r e s s u r e - te m p e r a tu r e conditions equivalent to both zeolite and greenschist facies. There are m inor occurren ces of A llam ooretype carb o n ate r o c k s, som e of which a r e a lte re d to talc, in the C a r r iz o Mountain Group. The carbonate rock s m ay have been o rigin ally inter stratifie d with the C arrizo Mountain sedim entary rocks. 4 Allamoore District The talc d istr ic t includes an a r e a about 20 m ile s long in an N W -SE. direction , and about 5 m ile s wide in an NE-SW . direction. The stratigraph ic and stru ctu ral relationships between the A llam oore and H azel F o r m a tio n s a r e co m p lex. It w as p re v io u sly su g g e ste d that the H azel F o r m a tion, with its conglom eratic portion derived from A llam oore rocks, w as tectoni cally m ixed with the A llam o o re F orm atio n in a com plex stru ctu re of folds and th ru sts (King, in King and Flaw n, 1953). H ow ever, rem apping in the d istric t s u g g e sts that the two fo rm atio n s w e re o rig in ally interbedded and that the ro ck s constitu te a sin g le fo rm atio n . In this tentative in terp retatio n , the d i s t r ic t is viewed a s an upside-down sequence that is of general hom oclinal form and that includes folds and faults. The carbonate ro ck s c o m p rise lim estone and dolom ite in varying proportions. L a y e r s with a high ratio of dolom ite to c a lc ite contain the m o st ch ert; v e r tic a l and la te ra l sedim en tary fac ie s changes in carbonate and ch ert la y e rs occur over short distances. Volcanic rocks include m afic flows and pyroclastic and volcaniclastic types. M afic flows now directly a sso c ia te d with other volcanic rocks cannot alw ays be distinguished from diabase sills. Alteration of mafic rocks is intense; alteration products include chlorite, m ica, calcite, dolomite, hem atite, and epidote. Talc Deposits T alcose rocks include talc (or talc rock) and talc-carbonate rock. Talc rock contains l e s s than 20 p ercen t carb o n ate m in e r a ls ; ta lc -c a rb o n a te ro ck s contain m o r e than 20 p ercen t carbo n ate m in e r a ls . T his div isio n roughly s e p a r a te s com m ercial (talc rock) from noncom m ercial (talc-carbonate) m aterial. 5 D e p o sits range fro m ta lc o s e s t r e a k s to zon es of ta lc o se ro c k s up to 600 feet wide and a m ile long. M ost are steeply inclined lens-shaped or tabular bodies. Som e have been contorted o r deform ed into iso clin al folds with adjacent carbonate and m afic rocks. M in erals of talcose rocks are m ostly talc and dolomite, with som e deposits containing both dolomite and calcite, or rarely dolomite and m agnesite. Sm all am ounts o f q u artz a r e w id e sp re a d . A m phibole and p oorly to w e ll- c r y s t a lliz e d la y e r s ilic a t e s o c c u r in v aryin g but g e n e ra lly m in o r quantities. Relict sedim entary features and incomplete replacem ent of some layers cle arly indicate that the talcose rocks w ere originally carbonate rocks. Internal structure of talc bodies is commonly m ore com plex than external form . The p u re st talc tends to be n ear the cen ter of talco se units, with in creasin g am ounts of talc-carbonate rock and lay ers of slightly altered and unaltered carbonate rock and chert toward the m argin s. F o lia tio n is w ell developed in m any talc d e p o sits and is g e n e ra lly p a r a lle l to o rigin al bedding of the host rock. W hereas talc w as deform ed largely by sh ear folding and differential movement along foliation planes, many included lay ers of carbonate rock and ch ert w ere folded and broken into segm en ts that w ere tectoni c a lly se p a r a te d in the talc. The sy ste m a tic reg io n al change of m etam o rp h ic g ra d e , together with the foliation of much of the talco se rock and the lineation of am phi boles in a sso c ia te d rocks d e scrib e d below, su g g e st that hydrotherm al m etam o rp h ism w as syntectonic. A lte r e d d ia b a se o c c u r s a d ja c e n t to o r in the vicin ity of talc d e p o sits and m ay have se rv e d a s a so u rce of heat in generating the h ydroth erm al solutions which selectively reacted with favorable carbonate lay ers. 6 ASBESTOS N early all of the w orld 's co m m ercially valuable a sb e sto s is ch rysotile and m o st of it o c c u rs in serpentinized u ltram afic igneous rock s. Am phibole a sb e sto s is found in a wide variety of igneous and m etam orphic ro ck s, including serpentinite, carbonate ro ck s, and iron form ations. Although its sh are of the com m ercial m a r k e t is s m a ll in c o m p a riso n to that of c h ry so tile , am phibole a s b e s to s h as many uses. The bundles of long, delicate fib ers of white asb e sto s from the A llam oore d istrict look very much like chrysotile, but x -ray diffraction studies and chem ical a n a ly s e s indicate that the a s b e s t o s is an am ph ibole type. In the d is c u s s io n that fo llo w s, the o c c u rre n c e of am p h ib oles and am phibole a s b e s t o s is c o n sid e re d in d etail a p p r o p r ia te to the p u r p o se of this r e p o r t, which is to b r ie fly d e s c r i b e the o ccu rren ce and geology of the a sb e sto s as a guide to further prospectin g. Blue Amphibole Rock B lu e a lk a li am phib ole and tre m o lite - a c tin o lite o c c u r in ro c k s within and n e a r talc bodies in som e a r e a s . The total am ount of rock containing blue am phibole is insignificant com pared with that containing talc; blue amphibole has not been noted at many deposits. The a lk a li am phibole v a r ie s fro m p ale blue to b la c k ; x - r a y d iffractio n and optical study su ggest that the am phibole m ay be m agn esio riebeck ite-rieb eckite of differing com positions. Albite and K -feld sp ar are apparently m ore abundant than the am ph ib oles but a r e difficult to reco g n ize in the field and m ay be m istak en for carbonate m in erals, with which they a r e intergrown. A sso ciated m in erals that gen erally occur in trace o r a c c e s s o r y am ounts include alk ali pyroxene, m ica, 7 chlorite, talc, tourmaline, rutile, pyrite, hem atite, and magnetite. The suite of m in e r a ls o c c u r s in differen t com binations and p ro p o rtio n s, and no constituent, including blue amphibole, is always present. A m p h ib o le s v a ry in fo r m fr o m fib ro u s to p r i s m a t i c and a r e contain ed fo r the m o st p art in carbonate ro ck s; som e thin la y e rs w ere p artially and selectiv ely m e ta so m a tiz e d to blue am phibole rock fo r tens o f feet. H ow ever, patchy r e p la c e m ents and thin irre g u la r veins larg e ly filled with slip -fib e r blue am phibole a r e the typical m odes of occurrence. F ib ro u s v arieties of m agn esio rieb eck ite-rieb eck ite a re known a s crocidolite and have been u sed a s a sb e sto s. H ow ever, the blue am phibole in the A llam o o re d istr ic t has only been found in sm all am ounts and gen erally m ixed with larg e quantities of im purities. Richterite-Talc Rock The white fibrous am phibole a sb e sto s p re se n t in the Diablo p ro sp e c t and adjacent a r e a s is rich terite, as shown by its ch em ical com position (table) and verified by com parison of x - ra y powder diffraction patterns with the calculated pattern for a K -rich terite from w estern A u stralia given by Borg and Smith (1969). R ichterite m ay be considered the alk ali-rich analog of trem olite, and its com po sitio n r e g a r d e d a s the r e s u l t o f su b stitu tin g two a lk a li ions fo r one of the two calcium ions of trem olite. Some richterite contains significant amounts of p o tassiu m , probably accom m odated by the relativ ely la rg e A stru ctu ral site. The 3. 54 p e rc e n t K^O in the Diablo p r o s p e c t am ph ibole is a h igh er p e rc e n ta g e than m ost rich terites contain. However, rich terites from volcanic rocks from M u rcia, Spain and the Leucite H ills, Wyoming (C arm ich ael, 1967), and from 8 W est K im b e r le y , w e s te r n A u s t r a l i a ( P r id e r , 1939) contain fr o m 4 . 4 to 6. 5 p e rc e n t K 2O. Such high concentrations of p o ta ssiu m m ay re p re se n t com plete filling of the A site (Papike et al. , 1969). Table. Chemical analysis of c r o ss fib er rich terite a sb e sto s fro m the Diablo p ro sp ect. (Collected by C. G. G roat; a n a ly s is by J . T . E th e r e d g e and D. A. Schofield, M ineral Studies Laboratory, Bureau of Economic Geology. ) Percent SiO z ....................................... . . A12 3 ................................... F eoO o (total F e ) ......................... . . . . T i 0 2 " . ................................... . . P 2 5 ....................................... C a O .................................................. . . . . M g O ................................................. . . M n O ................................................. N a ? 0 ................................................. k 2o ....................................... . . h 2o - ....................................... . . Ign. L o ss at 1050 C. . . . O O O 5 6 . 16 1.51 0.41 0.00 0.20 4 . 50 2 2 . 90 3.54 0.75 98. 53 N u m b e rs of ions on the b a s i s of 23 o xy g en s -- .................................. ..... 70 A 1 ................................... 0. 08 8 . 00 A 1 ....................................0. 17 F e .............................. 0 . 0 4 M g ..............................4 . 8 1 5.02 C a ................................... 0. 68 N a ........................._ . 1. 80 K .........................' . 0. 64 3. 12 9 Occurrences N orth-central part of district. -A s previously mentioned, only sm all amounts of im pure blue am phibole a sb e sto s have been found in the A llam o o re d istrict. M ost of the rich terite a sb e sto s d isco v e red to date is in a 2 - sq u a re -m ile a r e a of s m a l l h ills s e p a r a t e d by a llu v iu m (fig. 1). The carbonate ro ck s have been deform ed so that the com m on east-tren din g strik e and southw ard dip a r e now a n orth -tren d in g strik e and m o d e ra te to steep w estw ard dip. M afic igneous rocks are plentiful and many are probably diabase s ills. S ev eral sm all talc bodies and n um erous talc showings in outcrops a r e present. The m ain o ccu rren ce of rich terite is at the Diablo p ro sp ect, a low hill of d o lo m ite in which two l a y e r s of r i c h t e r i t e - t a l c ro c k w e r e e x p o s e d by tren ch in g (fig. 2). The two tren ch ed r i c h t e r it e - t a lc l a y e r s v a r y f r o m about 4 fe e t to p e r h a p s a s m u ch a s 10 f e e t th ick , and a r e a t l e a s t 135 and 60 f e e t lo n g, r e s p e c tively. A s b e s to s content v a r ie s fro m a tr a c e to about 75 p ercen t. The carb o n ate rocks contain e a sily recognizable blue am phibole fo r about 3 5 feet w est of the longest lay er. Blue amphibole fills thin irre g u lar fra c tu res, and intervening carbonate has been partially replaced by alkali feld spar and other m in erals. A sb e sto s o ccu rs as thin discontinuous se a m s and le n se s in g ray foliated talc, gen erally p a ra lle l o r at a low angle to the foliation of the talc. The fib e rs a r e up to 5 inches long and the h igh er the a s b e s t o s content of the talc ro c k , the g r e a t e r the length. R a r e ly a s b e s t o s o c c u r s in veins in thin dolom ite la y e r s in the talc. S lip -fib e r g en erally p red o m in ates but c r o s s - f i b e r and b en t-fib er fo rm s 10 100 150 Scale in feet ZOO 46504600- Alluvium Dolomite, cherty and calcar eous, locally includes some laicose layers Talcose dolomite and dolo mite with thin talc layers Richterite- talc rock-.richterite content highly variable includes some dolomite layers Approximate contact, dotted where inferred Strike and dip of bedding, with no implication as to original top Strike and dip of foliation Shallow trench Drill hole* Drill hole data 1-10% richterite >10% richterite F IG . 2. G eologic m ap and stru c tu re sectio n o f Diablo a s b e s t o s p r o sp e c t 11 a re also common. Some lay e rs of a sb e sto s have been sh ear folded and with su b se quent differential m ovem ent along foliation planes in the talc, the la y e r s of a sb e s to s w ere broken and separated . The com plex, sm a ll-sc a le structure is probably the result of deformation which preceded, accom panied, and followed form ation of asbestos. D rill holes in and around the hill penetrated a sb e sto s-b e a r in g talc la y e r s w est o f the s t r ik e lin e of the two e x p o se d l a y e r s . T h ic k n e ss of talc units r a n g e s up to about 20 feet; so m e include in te rla y e r e d dolom ite. L en gth s of talc units ran g e fro m a few feet to 200 o r m o r e a s su g g e ste d by drillin g and by an alogy with talc b o d ie s in o th er p a r t s of the d is t r ic t . A lt e r e d m a fic ro c k w a s p e n e tr a te d in two drill holes on the north side of the hill and one on the south side. Although there are probably five or m ore talc lay e rs, parts of which contain m o r e than 10 p e r c e n t a s b e s t o s and o t h e r s w hich c o n tain 1 to 10 p e r c e n t a s b e s t o s , the wide variation in dim ensions of individual la y e r s and the relativ ely widely sp ac e d d rill holes do not p e rm it a c c u ra te c o rre la tio n of units. F o r this re a so n , the s t r u c t u r e se c tio n (fig. 2) is not e n tir e ly a c c u r a t e . B e c a u s e of the e x t r e m e variation in a sb e sto s content within individual la y e r s , a unit showing only a trace of fib er w here in tersected in d rill holes m ay contain much m ore in its other p a rts. Some x - ra y diffraction patterns of apparently pure fiber show the p resen ce of sm all amounts of talc. This may represent a replacem ent of richterite during the final stag e of m e ta so m a tism . V ariab le but m inor am ounts of other m in e ra ls a r e c l o s e l y a s s o c i a t e d with the r i c h t e r i t e . T h e s e in clu de t a l c , t r e m o l i t e (? ), white m ica, dolomite, and a poorly crystallized layer silicate. These a c c e sso rie s a r e white o r r a r e ly pink, in m a rk e d distinction to the g ray talc ro ck in which 12 this second association of m inerals formed. The gray foliated talc itself consists of talc, dolomite, quartz, and poorly cry stallized layer silicate. In addition to the Diablo p r o s p e c t th ere a r e r e p o r t s of four oth er s e p a r a te o c c u rre n c e s of rich terite a sb e sto s in the A llam o o re d istric t; three of these d e sc rib e tr a c e s of white a sb e sto s noted in d rill cuttings fro m thin talco se la y e rs. A bout 1, 200 fe e t w e s t- s o u t h w e s t o f the Diablo p r o s p e c t , a 6 5 -fo o t- d e e p d r ill hole n ear the b a se of a sm all hill of vertical-bedded dolom ite penetrated talc with 75 p e rc e n t lo n g -fib e r ric h te rite a s b e s t o s o c c u rrin g betw een 38 and 60 feet. Sev eral shallow trenches near the hole exposed talc-carbon ate rock. Some blue am phibole rock is scattered along the c r e s t of the hill. S ev eral other holes w ere drilled and although som e talc w as in tersected, asb e sto s w as not found. Other p arts of d istric t. -M inor amounts of white asb e sto s w ere found during d r illin g and d e v elo p m en t of the r e la tiv e ly l a r g e talc body of the B u c k m in e (fig. 1). S e v e r a l s a m p l e s show ed the a s b e s t o s to be c r o s s - f i b e r s e a m s up to about two inches thick and at le a st six inches long,which are concordant with the foliation o f the talc. V ery thin p artin g s o f talc a r e both roughly p a r a lle l and oblique to the m argin s of the a sb e sto s la y e rs. B en t-fiber asb e sto s and other deform ational stru ctu res such as those found at the Diablo p ro sp ect a re absent. The asb e sto s in the B uck m in e a r e a is r ic h te rite and is s i m i l a r to that at the Diablo p r o sp e c t and nearby occurren ces. X -ray diffraction analysis indicated that what appears to be pure fib er actually contains a sm a ll amount of talc. A little white asb esto s w as reportedly observed during extraction of talc at the T. & P. No. 1 m ine. L a t e r , d r ill cuttin gs containing f ib e r s up to about h alf an inch long w ere obtained. The fiber is rich terite, although m o re brittle than in 13 the other o c c u r r e n c e s. What a p p e a rs to be pure fib er contains so m e ch lorite, perhaps as pseudomorphous replacement. T here is an unconfirm ed report of white asb e sto s from the relatively sm all talc body of the N eal-M ann p ro sp ect (fig. 1). V ery sm a ll m a s s e s of n early pure asb estifo rm actinolite also occur within the talc. ORIGIN AND GEOLOGIC SIGNIFICANCE Blue am phibole and ric h te rite -ta lc rock s a r e thought to be con cen tration s of constituents, mainly aluminum, iron, sodium, and potassium , originally present a s m inor im p u rities in carbonate rocks that have been rep laced by talc. During m e ta so m a tism , these constituents f ir s t m ay have been incorporated into new fo rm s such a s lay er silic a te s, then m obilized and either concentrated in carbonate la y e r s , in another talc body, or in another p art of a single talc body. F orm atio n of r ic h te r ite - ta lc rock a lso involved con cen tration of a lk a lie s re la tiv e to alum in um and iron. The effect of stru ctu ral co n tro ls, though perh aps im portant, is unknown. B ecau se the im purities m ay have constituted only a sm all percen tage of the origin al carbonate rock, concentrations of these elem ents such a s at the Diablo prospect su g gest that much m o re talc m ay occur in the vicinity. R ich terites have been recognized in a wide variety of ro ck s, but unlike the relatively low-tem perature Allam oore richterite, m ost occurrences are suggestive of re la tiv e ly high te m p e r a tu r e s of fo rm a tio n . In addition, m any r ic h te r ite s a r e c o m p o s it io n a lly in t e r m e d ia t e b etw een a l k a l i and c a l c i u m a m p h i b o l e s , w ith 1. 00 < C a < l . 50 p e r f o r m u la unit, f u r t h e r s u g g e s t i n g h ig h - t e m p e r a t u r e h y p e r s o l v u s conditions (E rnst, 1968). However, rich terites with calcium contents of le ss than 14 about one calcium per form ula unit, such as the Diablo p rosp ect am phibole, m ay be co n sid ered sim ply as alkali am phiboles and m ay be reason ab ly expected to fo rm at relatively low tem peratures. Exam ples of asbestiform richterite are rare; however, greenish-gray r i c h t e r i t e a s b e s t o s w ith c a l c i u m content o f 1. 03 p e r f o r m u l a unit, both d i s s e m i n ated and o ccu rrin g a s c r o s s - f i b e r vein s up to 5 cm thick, h as been rep o rted in trem o lite-actin o lite-b earin g ro ck s and in other lithologies in the Aldan region of S ib eria (Ivanov and Sidorenko, 1965). Although rock com positions rich in alk alie s relativ e to alum inum a r e also com p aratively r a re and preclude the com m on o c c u r r e n c e o f r i c h t e r i t e s , the l a c k o f l o w - t e m p e r a t u r e e x a m p l e s --e s p e c i a l l y a s b e s t if o r m o n e s - m a y be due in p a r t to incom plete and in a ccu rate iden tificatio n s. F o r exam p le, veins of c r o s s - f i b e r a s b e s t o s s im ila r in a p p e a ran ce to ch ry so tile and occurrin g in serpentinite w ere d escrib ed as "so d a -ric h anthophyllite" by L a u d e r m i l k and W ood ford (1930). H o w e v e r, the a m p h ib o le c o n ta in s 5. 10 p e r c e n t CaO and 7. 40 percen t Na^O, suggestin g that it m ay be rich terite. An a s s e m b la g e of ro c k s s i m i l a r to those in the A lla m o o r e d is t r ic t o c c u r s in the R obertstow n-T ruro a re a of southern A u stralia. M agnesian crocidolite occurs in talco se dolom ite and altered dolerite (Wymond and W ilson, 1951), and alkalirich tre m o lite a s b e s to s in the ta lc o se dolom ite h as a ls o been p ro sp e c te d (D. King, 195 5). The p r e se n c e of la r g e talc r e s e r v e s in the a r e a w as recen tly announced (Industrial M in erals, 1970, 1971). 15 POTENTIAL F ib e r s of much of the rich terite a sb e sto s fro m the A llam o o re d istr ic t a re g re a te r than one inch in length, have very sm a ll d ia m e te rs, and have relatively high tensile strength for am phibole fib e rs. The a sb e sto s is white to light gray ; so m e is g re e n ish or r a r e ly pink. When se p a ra te d , the fib e r s fo rm a white, s e m i h a r sh , fluffy m a s s . The ric h te rite m ay be s i m i l a r in its p r o p e r tie s to tre m o lite a s b e sto s which has been used in a wide variety of products; how ever, in devices like ch em ical filte rs, a pure product is required. Although the a sb e sto s can be s e p a r a te d e a s ily fro m m o st of the ta lc , it m igh t prove d ifficu lt to re m o v e a ll the v e ry fin e - g r a in e d platy p a r t ic le s of t a lc . In c o n t r a s t to o th er types of a s b e s t o s , the richterite m ay have talc as a byproduct or coproduct. P relim in ary work at the Diablo prosp ect su ggests probable r e se r v e s of at l e a s t 5, 000 ton s of a s b e s t o s in r o c k c o n tain in g 10 p e r c e n t o r g r e a t e r f i b e r . How e v e r, m o re c lo se ly sp aced d rillin g is req u ired to e stab lish proved r e s e r v e s , and might show considerably m ore asb e sto s. This includes additional asb esto s at g r e a t e r depth a s downdip e x ten sio n s of ro c k s known to contain r ic h te r ite - ta lc la y e r s , and a lso in d e p o sits im m ed iately o u tsid e the d rille d a r e a . P ro sp ectin g p o ssib ilitie s a r e excellent in the a re a surrounding the hill w estsouthw est of the Diablo p ro sp ect, w here there w as a good showing of a sb e sto s in a d rill hole. T here a r e about four sq u are m ile s in the n orth -cen tral p art of the d istrict where m etasom atic rocks are known and where co m m ercial am ounts of asso ciated richterite asb esto s might be found. Estim ation of possible re se rv e s fo r this a re a , including the Diablo prospect, is speculative; 50, 000 tons o r m ore 16 m igh t be p ro ved by an adequate exp lo ratio n p r o g r a m . In o r d e r to te st this p o s s i bility, d rillin g through alluvium would be n e c e s s a r y ; depth to bed ro ck in m uch of this a r e a p ro bably ra n g e s fro m a few feet to about 30 feet. The very sm all amounts of asb e sto s found at the Buck and T. & P. No. 1 m in es, and N eal-M ann prospect, considering the drilling and talc extraction already accom plished, suggest that further testing for asb esto s alone might not be profitable. REFERENCES B o r g , I. Y. , and Sm ith, D. K. , 1969, C alcu lated x - r a y powder p attern s for silic a te m in e r a ls : Geol. Soc. A m e ric a M em . 122, p. 382-387. C arm ich ae l, I. S. E. , 1967, The m in eralogy and petrology of the volcanic rocks fro m the L eu cite H ills, W yoming: Contr. M in eralo g y P etrolo g y , v. 15, p. 24-66. E r n s t , W. G. , 1968, A m p h ib o le s : N ew Y o rk , S p r i n g e r - V e r l a g New Y o r k , I n c . , p. 29. Flawn, P. T. , 1958, T exas m in ers boost talc output: Eng. Mining Jour. , v. 159, p. 104-105. Industrial M in erals, 1970: London, M etal Bulletin, L td ., no. 33, p. 37. ____________ , 1971: London, M e ta l B u lle t in , L t d . , no. 45, p. 29. Ivanov, I. P. , and Sidorenko, G. A. , 1965, R ich terite a s b e s to s : All-U nion Min. Soc. M e m ., v. 94, p. 497-506. 17 King, D . , 1955, Investigation of a sb e sto s d ep o sits in the Robertstow n and T ru ro d istricts: Dept. M ines, South A u stralia, Mining Rev. , no. 103, p. 58-73. King, P. B. , 1965, Geology of the S ie rra Diablo region, T ex as: U. S. Geol. Survey P ro f. P ap er 480, 185 p. ____________ , and F la w n , P . T . , 1953, G e o lo g y an d m i n e r a l d e p o s i t s o f P r e cam brian rocks of the Van Horn a re a , T exas: Univ. T exas Pub. 5301, 218 p. L a u d e rm ilk , J . D. , and W oodford, O. A . , 1930, S o d a-ric h anthophyllite a s b e s to s fro m Trinity County, C aliforn ia: A m e r. M in e ralo g ist, v. 15, no. 7, p. 259-262. Papike, J. J. , R o ss, M. , and Clark, J. R. , 1969, C rystal-ch em ical ch arac terization of clinoam phiboles based on five new structure refinem ents: M in eralo g ical Soc. A m e ric a , Spec. Pub. 2, p. 117-136. P rid e r, R. T. , 1939, Some m in erals from the leu cite-rich rocks of the W est K im berley a re a , w estern A u stralia: Mining M ag. , v. 25, p. 373. Wymond, A. P. , and W ilson, R. B. , 1951, An occurren ce of crocidolite near Robertstown, South A u stralia: T ran s. Royal Soc. South A u stralia, v. 74, p. 44-48.